Critical Boundaries in Earth's History - and the K-T Boundary

Contributed by the ODP Leg 171B Shipboard Scientific Party
Ocean Drilling Program Leg 171B was designed to recover a series of 'critical boundaries' in Earth history in which abrupt changes in climate and oceanography coincide with often drastic changes in the Earth's biota. Some of these events such as the Cretaceous-Paleogene (K-T) extinction and the late Eocene tektite layers are associated with the impacts of extraterrestrial objects, like asteroids or comets, whereas other events, including the benthic foraminifer extinction in the late Paleocene and the mid Maastrichtian extinction events, are probably related to intrinsic features of the Earth's climate system. Two of the critical boundaries, the early Eocene and the late Albian, are intervals of unusually warm climatic conditions when the Earth is thought to have experienced such extreme warmth that the episodes are sometimes described as 'super-greenhouse' periods. The major objectives of Leg 171B were to recover records of these critical boundaries at shallow burial depth where microfossil and lithological information would be well preserved, and to drill cores along a depth transect where the vertical structure of the oceans during the boundary events could be studied.

Five sites were drilled down the spine of Blake Nose, a salient on the margin of the Blake Plateau where Paleogene and Cretaceous sediments have never been deeply buried by younger deposits. The Blake Nose is a gentle ramp extending from about 1000 m water depth to about 2700 m depth, and is covered by a drape of Paleogene and Cretaceous strata that are largely protected from erosion by a thin veneer of manganiferous sand and nodules. A continuous, expanded, and almost complete, record of the Eocene period was recovered that shows Milankovitch-related cyclicity. In combination with the excellent magnetostratigraphic record and the presence of both calcareous and siliceous microfossils, the Milankovitch-controlled cycles will be used to recalibrate the late-middle Eocene and late Eocene timescale. Radiometric dates on ash layers, and dating by astronomical tuning, will produce an integrated timescale to recalibrate magneto- and biostratigraphy. In addition, the chemistry of the well-preserved calcareous microfossils will be used to document climate variability when the Earth's climate switched from a greenhouse to an icehouse state.

Leg 171B recovered a suite of critical events in Earth's history that includes the late Eocene radiolarian extinction, late Paleocene benthic extinction, the K-T boundary, the mid Maastrichtian event, and several episodes of organic-rich sediments in the Albian warm period. The upper Paleocene benthic foraminifer extinction occurs within an expanded interval of calcareous sediments unlike most regions of the Atlantic where calcareous fossils have been severely dissolved just above the extinction horizon.

The recovery of spectacular records of the K-T boundary attracted the attention and imagination of both the public and the international news media. The K-T boundary was recovered at three sites, each with a biostratigraphically and magnetostratigraphically complete sequence that includes the earliest part of the aftermath of the Late Cretaceous extinctions. Three copies of the boundary interval at one site were collected in a section that includes a 10-17 cm thick graded bed of green spherules capped by a fine-grained, rusty brown limonitic layer that is overlain by dark gray clay of the earliest Danian. This succession is interpreted as fallout from the Chicxulub impact structure on the Yucatan Peninsula and the succeeding deposition of lowermost Danian sediment following the K-T extinction event. Notably, neither of the two K-T boundary sections drilled updip from this site have well developed ejecta beds between earliest Danian and latest Maastrichtian deposits. The spherules at these sites were either slumped into deeper water very shortly after deposition or turbidites carrying the ejecta debris bypassed the upper slope and deposited at least part of their load near the tip of Blake Nose. The recovered sections of the K-T boundary are complete at several sites, and thus excellent for studying the response of marine biota to the extraterrestrial event. For instance, the planktonic foraminifera are extremely well preserved, and hence are ideal for stable isotope studies that hopefully will reveal the chain of climate events caused by the impact.

Based on the results from drilling, the shipboard scientific party has derived a preliminary geologic history of Blake Nose. The Blake Nose is composed largely of Jurassic to mid Cretaceous carbonate platform deposits. The platform rests on basement rocks formed by intrusion and volcanism through attenuated continental crust during the rifting stage of the Atlantic. As much as 10 km of carbonates accumulated in this area. By about 110 Ma, the reef tract stepped back 40-50 km from the lower Cretaceous margin and formed a long tract of coral-rudist reefs. These reefs ceased growth a few Ma later, and the deposition of green and red variegated clays began. Black shales of latest Aptian age (about 105 Ma) found at Site 1049 suggest that the disaerobic conditions associated with the organic rich sediments extended to a water depth of at least 1500 m. Black shale deposition returned in the late Albian-Cenomanian in a series of cycles that are age-correlative with Oceanic Anoxic Event 1d known from Europe and elsewhere.

There is a widespread, major unconformity above the Cenomanian (100 Ma) on Blake Nose from which upper Cenomanian to lower Campanian (95-75 Ma) strata were largely removed. In addition, the Maastrichtian sequence (65-70 Ma) contains numerous slumps, including one at the Maastrichtian-Cenomanian contact at Site 1052, so it is possible that Campanian sediments were removed from the area of Site 1052 by down slope transport. Despite the slumping, much of the Maastrichtian appears to be present as a drape of nannofossil chalk and ooze. The preserved record has a well-developed color banding that may record orbital cycles.

The end of the Cretaceous and earliest events of the Cenozoic (K-T boundary) are well preserved on Blake Nose. Deposition of a nearly uniform drape of pelagic sediment continued into the Paleocene. Paleocene strata are the first to preserve geochemical and lithological evidence for abundant volcanic ash on Blake Nose - a trend that continued throughout the Eocene.

By the latest Paleocene, deposition was concentrated into a major clinoform stack that reached its greatest thickness near the center of the Blake Nose transect. At least two hiatuses are present in this sequence. One is within the uppermost Paleocene, and occurs close to the upper Paleocene 'Thermal Maximum' when the deep oceans appear to have abruptly warmed for a few hundred thousand years. However, the hiatus is either absent or very short near the center of the clinoform stack where the Paleocene-Eocene transition is biostratigraphically complete. A second is present near the lower-middle Eocene transition where about two million years of the Eocene are absent across the whole of Blake Nose. The locus of sedimentation apparently backstepped up the slope of Blake Nose during the middle and late Eocene. Sediments are mostly green siliceous nanofossil chalks and ooze with well-preserved calcareous and siliceous microfossils. Volcanic ash beds are common throughout the sequence and probably record major eruptions in the Lesser Antillies. The combination of good magnetostratigraphy, biostratigraphy, and color cycles should result in great improvements in the chronology for part of the Eocene. In addition, sediments correlative with the Upper Eocene meteorite impact event in the Chesapeake Bay were recovered.

It is probably no coincidence that the youngest Eocene sediments are of latest Eocene age. The Oligocene is associated with widespread hiatuses in North Atlantic. The Gulf Stream assumed its present course for the most part in the Oligocene and cut into the surface of the Florida Straits and the Blake Plateau. A highstand of sea level in the late Oligocene shifted sedimentation from the shelf to the coastal plain starving the outer shelf slope landward of the Blake Escarpment. In the Blake Basin, Oligocene cooling at high latitudes intensified the southward flow of water along the Blake Escarpment and formed the widespread seismic reflector that represents an unconformity distributed over most the western North Atlantic.

Shore-based research will focus on studies of the sequence of events surrounding both the K-T and Upper Eocene impact events and the oceanographic history of the Paleogene and Cretaceous sequences. The excellent magnetostratigraphy record from Blake Nose will be used to determine the polar-wander path of North America from the Aptian to the Eocene. Major research effort will be devoted to analysis of history of orbital forcing of both Cretaceous and Cenozoic climate and the dynamics of both the Cenomanian and lower Eocene warm periods. Finally, the depth transect of cores will be used to reconstruct the vertical structure of the oceans in the distant past when patterns of ocean circulation were much different from today.